Intermediate Phase Assisted Preparation Of Carbon-electrode All-inorganic Mixed-halide Perovskite Solar Cells

The all-inorganic lead-halide perovskite CsPbI_xBr_3-x（0≤x≤3）is a promising photovoltaic material due to its excellent optoelectronic properties and thermal stability.Combined with screen-printed composite carbon materials,carbon electrode all-inorganic perovskite solar cells with many potential advantages such as low cost and excellent stability have become one of the hotspots in the field of perovskite photovoltaic research.The preparation of CsPbI_xBr_3-x films with high-quality,phase-stable,and excellent moisture resistance is the top priority for realizing high-efficiency and stable carbon-electrode all-inorganic perovskite solar cells,and it is also one of the current challenges.In this paper,an innovative research idea to promote film growth by intermediate phase assisted is proposed.A method for preparing CsPbIBr₂ films by in-situ polymer-covered annealing is proposed.And an intermediate phase assisted sequential deposition for desired CsPbI₂Br films is proposed.Based on the detailed study of the film growth process and physical laws,CsPbIBr₂ and CsPbI₂Br films with high crystalline quality and low defect density were obtained,and the halogen phase separation of the CsPbIBr₂ film was suppressed.The phase stability of the CsPbI₂Br films in a high-humidity environment is improved,and the efficiency and stability of the carbon-electrode all-inorganic perovskite solar cell are effectively improved.The main contents of this study are as follows:1)An in-situ polymer-covered annealing strategy was proposed to improve the traditional one-step solution method to control the crystallization process of CsPbIBr₂ films.By spin-coating poly（methyl methacrylate）（PMMA）solution on the CsPbIBr₂ precursor film before annealing,a layer of in-situ polymer is covered on the surface of the precursor film to slow down the growth rate of grains.By optimizing the concentration of PMMA solution,a CsPbIBr₂ film with an average grain size of 626 nm,[110]-preferred orientation,reduced halide segregation,stoichiometric composition,and decreased non-radiative recombination can be acquired,which lead to the improved transport and suppressed non-radiative recombination of charge carriers for the resulting PSC.Hence,the optimum carbon-electrode PSC exhibits a significantly enhanced efficiency from 8.71%to 10.50%,which is better than that of nearly all the CsPbIBr₂ PSCs with the same configuration reported earlier2)It is realized by first forming a Pb Br₂ film onto FTO/Ti O₂ substrate by spin-coating and then spin-coating of Cs I water solution（Cs I/H₂O）onto it.After the two-step spin-coating procedures,the Ruddlesden–Popper（R–P）perovskite intermediate phase film composed of Cs-Pb-I-Br complex is formed,which can be further transformed into CsPbI₂Br grains and Cs Br species by annealing through spinodal decomposition reaction.The resulting CsPbI₂Br film is featured with full coverage,microsized grains,and excellent phase stability in ambient air with controlled relative humidity（RH）of 60～70%.Moreover,the by-product Cs Br species located at grain boundaries can effectively passivate the defects,thus weakening the non-radiative recombination and boosting the transportation property of charge carriers.As a result,the carbon-electrode CsPbI₂Br PSC yields both record-high PCE of 15.24%and V_oc of 1.312 V,along with superior humidity stability.